United States federal regulations require that the functions of all emission-relevant components (such as injection systems, catalytic converters and the like) of a motor vehicle must be monitored during the operation thereof utilizing on-board means (on-board diagnosis, OBD). For vehicles having internal combustion engines including diesel vehicles, oxidation catalytic converters as well as NOx catalytic converters (so-called denox catalytic converters) are utilized for improving exhaust-gas values. Hydrocarbon sensors (HC sensors) are utilized to monitor the operation of such catalytic converters. These sensors are based essentially on the mixture potential principle and are similar to the lambda probes known per se with respect to their configuration. With an otherwise same configuration, these HC sensors differ from the lambda probe sensors essentially only in the hydrocarbon sensitive electrode coating.
The hydrocarbon concentration in the exhaust gas of an internal combustion engine can be detected by means of these hydrocarbon sensors and from this, a conclusion can be drawn as to the operability, for example, of an oxidation catalytic converter or a denox catalytic converter.
It is problematic with such hydrocarbon sensors that a considerable loss in sensitivity occurs during long-term operation. For a long service life, tolerances develop which are so great that monitoring of the catalytic converters in the context of the OBD is no longer possible. Accordingly, either the deterioration must somehow be prevented or the loss of sensitivity occurring because of deterioration must be compensated in some way in order to make such hydrocarbon sensors more stable for a long period of time.
In a known method for determining the sensitivity of a hydrocarbon sensor, measured values, which are detected by the hydrocarbon sensor and are made erroneous because of its deterioration, are corrected via an elapsed-time counter and a previously determined deterioration characteristic line. For example, for an exchange of the control apparatus and/or of the sensor, it is here however disadvantageous that the elapsed-time counter must always be brought to the correct count which is associated with a considerable complexity.
In another known method, a calibration is carried out for known gas concentrations during operation. This, however, is not possible during driving operation because the emissions of the engine change in the course of time and therefore cannot define a reference.